Circumferential grooves and other features such as shoulders and beads may be formed in pipe elements by various methods, one of particular interest being roll grooving. As shown in FIG. 1, roll grooving methods involve engaging an inner roller 10 with an inner surface 12 of a pipe element 14 and an outer roller 16 with an outer surface 18 of the pipe element opposite to the inner roller and incrementally compressing the sidewall 20 of the pipe element 14 between the rollers 10 and 16 while rotating at least one of the rollers. Rotation of one roller (often the inner roller) causes relative rotation between the roller set and the pipe element, and features on the inner and outer rollers form corresponding features on the inner and outer surfaces of the pipe element. In one example roll grooving method the rollers remain in a fixed location and the pipe element rotates about its longitudinal axis relative to the rollers. In another example embodiment the pipe element remains stationary and the roller set traverses the pipe element's circumference.
During roll formation of circumferential grooves in pipe elements a problem may arise if the direction of the force applied to the pipe element by the outer roller is not substantially perpendicular to the longitudinal axis 22 of the pipe element 14 (i.e., the rotational axis of the pipe element). This may occur for example, when forming a circumferential groove 24 having an asymmetrical cross sectional shape, as shown in FIGS. 1 and 1A. Although the force 26 applied to outer roller 16 during groove formation is substantially perpendicular to the rotational axis 22 of the pipe element 14, interaction between the pipe element 14 and the angularly oriented face 28 of the raised surface feature 30 of the outer roller 16 imparts an axial force component 32 (i.e. parallel to the axis 22) to the pipe element 14. The axial force component 32 is not counteracted due to the asymmetry of the raised surface feature 30, the face 34 opposite angular face 28 being oriented substantially perpendicularly to the rotational axis 22 of the pipe element 14 and the intermediate face 35 (extending between faces 28 and 34) being substantially parallel to the rotational axis 22. The geometry of faces 34 and 35 render them unable to counteract the axial force component 32. The axial force component 32 increases the force between perpendicular face 34 of the outer roller 16 and the pipe element 14, specifically, between face 34 and the side surface 36 of groove 24. As the groove 24 is formed the increased force between face 34 and the side surface 36 of the groove 24 increases the friction between these surfaces. It is believed that this increase in friction between face 34 and the side surface 36 causes the intermediate surface 35 of outer roller 16 to slip relatively to the floor surface 37 of the groove 24. This slippage between intermediate surface 35 of the raised surface feature 30 and the floor surface 37 of groove 24 makes it difficult to use the rotation of outer roller 16 to measure and thereby accurately form a groove 24 having a desired circumference as measured at the groove floor surface 37. (Note that the circumference of the pipe element, or a groove therein, may be calculated knowing the diameter of the outer roller and the number of rotations it experiences while traversing the pipe element circumference without slipping relative to the pipe element.) There is clearly an opportunity to improve outer rollers for roll grooving that will permit accurate measuring and monitoring of the circumferential groove having an asymmetrical cross sectional shape during groove formation.